Stepper motors are used in a variety of paper handling equipment. For example, in a conventional printer, stepper motors are used to move paper through the printing mechanism. Also, in a conventional plotter, two stepper motors are used to move the plotting pen to plot an image on paper.
A stepper motor is ordinarily operated with only gradual changes in angular velocity to avoid a motor stall. Such a motor is driven by two or more bilevel signals. In response to each level change of a drive signal, the motor turns its rotor an incremental angle. A motor is said to stall when its rotor fails to advance as directed by a motor drive signal. The motor will not stall if a level change is received when the rotor has turned enough to enter a limited angular range. If a level change is received before the rotor has entered this angular range, or after the rotor has turned beyond the range, the motor may stall.
A motor stall in a printer, for example, interrupts the printing process. It may then become necessary for an operator to remove partially printed paper and restart the printing process. Such intervention by an operator adds delay and increases the cost of operating the printer and the associated computer system.